And now, our washington watchdog is in action, because just this week, a new law on guns and background checks began moving toward a possible vote. A bill, a compromise under way. And tonight, abc's david kerley has found something in the fine print that deserves a headline. Here's david on the case. Reporter: It is the number one murder weapon in the country. This handgun used to kill a man in new york. To help the nypd track down the killer to solve the murder, the history of this gun took the same trip we did, to this undisclosed building. Where we were given access to one of the most amazing vaults in the country. 12,000 weapons, rifles, even an anti-tank cannon. Somebody had this and we seized it. Reporter: This is the atf's library of weapons. Where felts try to help local police solve a crime, figure out the exact type of gun and trace where it was bought. That means paperwork. And here, they are forced by congress to do things the old fashioned way. By hand. While a buyer's background may have been checked by the fbi, the only record of a sale is a gun dealer's form on paper, not computerized. A half billion paper records transferred to microfilm are now scanned. The inefficiency of this labor intensive antiquated system cost taxpayers $60 million the past 12 years. That's a lot of time. That's a lot of money. Yeah. When people are visually going through microfilm, searching through 60,000 pages of information for a single gun, they can miss it. Reporter: If all this was computerized, a trace would take a matter of hours. It now takes about five days. Why no computers? Because members of congress, backed by the nra, passed laws forbidding the agency from kussing computers that can read the content of the forms, creating a registry. Gun supporter senator charles grassley was instrumental in passing these laws. Does that make sense when we're trying to fight crime? Under the second amendment, i don't want the government registering guns. I don't want the government knowing who has a gun and who doesn't have a gun. Reporter: So, aren't you actually costing me money by not allowing the atf to do their job? We go out of our way to protect constitutional rights in this country. Reporter: So, in this high tech age, connecting guns with killers is stick in the last century. David kerley, abc news, at the atf tracing center.

This transcript has been automatically generated and may not be 100% accurate.

{"id":18944488,"title":"Tracing Guns Used in Crimes","duration":"2:32","description":"Why tracking down criminals who use firearms is so difficult in the modern world.","url":"/WNT/video/tracing-guns-crimes-18944488","section":"WNT","mediaType":"default"}